Put fish, onion, seasoning and water into a large pan and bring to boil. Cover and simmer for 20 minutes. Meanwhile peel, boil and mash potatoes.

Lift out fish and leave to cool for a minute, skin and flake. Strain the stock and return to the pan. Add flaked fish, milk, butter and potato. Bring to boil and simmer for a few minutes. Correct seasoning and serve garnished with a little chopped parsley and a spoonful of cream per person.